1. Field of the Invention
The invention relates to the treatment of subterranean formations in order to increase productivity or injectivity of such formations by an improved fracture acidizing technique.
2. Description of the Prior Art
Many petroleum-containing reservoirs are composed of limestone, dolomite or sandstone of low permeability. Although limestone and dolomite formations may contain large quantities of petroleum materials their permeabilities are generally quite low so that the oil production rates from them may be uneconomic because of resistance of the formations to fluid flow. Areas in the formation a substantial distance from the producing wells penetrating the formation may not be drained at all. A common method that has been used in an effort to stimulate production from such formations has been acidization. That is, acid is generally pumped down the well and displaced into the formation to dissolve a part of the formation and create channels of increased permeability thereby increasing fluid conductivity in the formation. Limestone or dolomite formations are generally best treated with hydrochloric acid or a similar acid and sandstone formations are treated with a mixture of hydrochloric and hydrofluoric acid. It is often necessary to pump the acid into the formation at a rate that will fracture the formation thus conducting it rapidly therein to avoid expending the acid in the area near the well bore. If the acid is expended only around the well bore it will have no beneficial effect in increasing transmissibility of fluid present a substantial distance from the well bore.
However, the presence of fractures extending from the well into the surrounding formations does not necessarily greatly increase the amount of acid that reaches substantial distances from the well bore before being spent by reaction with the formation. The high temperature of most subterranean formations and the extremely rapid reaction of the mineral acids which are used causes most of the acid to be neutralized by reaction with the calcareous formation before it can traverse a great distance into the formation from the well bore. One prior art method that has been used in an effort to acidize fractures more effectively has been to protect the faces of the fractures by coating them with a material such as a gel that will be slowly destroyed by the acid as it is displaced into the fracture. However, these coatings are only partially effective because the coating on that portion of the fracture closest to the well is the first to be contacted with acid and destroyed. Acid subsequently displaced into the fracture passes and contacts the exposed surface of the formation close to the well and is largely spent so that insufficient acid reaches the outer parts of the fracture. Similarly, sometimes fluid-loss reducing additives such as fine silica flour are added to control leak-off but the acid can quickly sweep the flour away and reacts rapidly with the thus exposed calcareous surface channels near the borehole so that much of the acid is again lost before reaching the outer limits of the fracture.
The difficulties in acidizing fractures at substantial distances from the well are aggravated in hot carbonate formations because the acid treatment methods heretofore available have resulted in little more than borehole enlargement at the high rate of reaction at the elevated temperature existing in such formation. The elevated temperature existing in hot carbonate reservoirs also aggravates the problem of inhibiting hydrochloric acid to prevent excessive corrosion by the acid of the casing and tubing in a well. Efforts have been made to pump large volumes of water ahead of the acid to cool the tubing and fracture faces. However, this procedure is not totally dependable and it is generally not desirable to pump large quantities of water into productive formations and especially not into gas reservoirs. U.S. Pat. No. 3,707,192 attempts to solve this problem by a multiple stage injection procedure whereby two substances are pumped into the formation sequentially; the second reacting with the first to form an acid capable of reacting with at least a portion of the formation. This method, however, requires two reactants to mix within the fracture to produce the acid. Such a process would be wasteful and a high efficiency of mixing would be unexpected in porous media. More importantly, the cited reactions would be expected to be very rapid and thus spent very quickly as in most other prior art processes.
It is an object of my invention to solve these problems by using an acid which will slowly solubilize in an aqueous medium. This acid in solid form may be pumped into the formation through fracture fissures to great distances from the borehole where it then slowly dissolves thereby removing the problem of rapid reaction near the well bore and effecting continuous etching of the fracture surface which is known to greatly improve treatment effectiveness by establishing communication between well bore and the extremity of the fracture.
A further object is to utilize the solid acid to prop the created fracture during the initial stages of production while the acid is undergoing dissolution.